1. Field of the Invention
This invention relates to the growth of IVA-VIA compounds and, more particularly, to the growth of composites comprising layers of IVA-VIA compounds and alloys on substrates.
2. Description of the Prior Art
One application of IVA-VIA compounds is to infrared (IR) detectors. To date, various semiconductor materials have been used to form IR detectors, with the result that such detectors presently cover a wide range of the IR spectrum. However, past efforts have been concentrated on extrinsic semiconductors, rather than intrinsic semiconductors, because of the highly advanced technology for these materials, which is primarily the result of transistor technology.
While detectors formed from extrinsic semiconductors can exhibit high D* and high speed, they are generally limited to operation at very low temperatures. Also, a relatively large volume of detector material is usually required in order to absorb sufficient incident radiation for detection. In addition to the obvious restrictions on the miniaturization of detector arrays and the accompanying signal processing circuitry, this large volume causes radiation hardening problems.
In principle, compound semiconductor materials, which are used for most intrinsic detectors, have all the advantages of the elemental (extrinsic) semiconductors and very few of the disadvantages. Pb.sub.1-x Sn.sub.x Te (lead tin telluride) and PbTe (lead telluride; x=0) are examples of excellent compound/alloy detectors for long wavelength IR. Lead tin telluride, for example, has several advantageous characteristics. Because of its direct energy gap, lead tin telluride does not required impurity to absorb incident photons, with the result that there are additional 10.sup.4 cm.sup.-3 available sites for capturing incident signals. Satisfactory operation with relatively thin detector films on the order of 10 .mu.m thickness is thus possible. Also, the composition of lead tin telluride can be altered for tuning to the desired portion of the IR spectrum. A composition of approximately Pb.sub.0.8 Sn.sub.0.2 Te (x = 0.2) is particularly useful because junction-type photovoltaic detectors using films of this composition provide peak response to infrared radiation in the 10-12 .mu.m region.
Some growth techniques which have been used successfully for the formation of single crystal IVA-VIA compounds and alloys on other IVA-VIA compounds and alloys have not been successfully applied to insulating substrates. For example, liquid phase epitaxy (LPE) has not been reported in the growth of IVA-VIA compounds on insulators such as the alkali halides and oxide insulators, presumably because of problems in properly nucleating the IVA-VIA material. Yet there are considerable reports of the use of evaporation and sputtering for growing IVA-VIA materials on insulating halide substrates. This area is reviewed by H. M. Manasevit in J. Crystal Growth, vol. 22, pg. 125 et seq. (1974). It should be noted that these processes lead to slow growth rates on insulators, typically less than about 350 A/min. (0.035 micron/min.). This is particularly disadvantageous and impractical for applications where thick films may be required, as in IR detectors. In addition, the composition of the grown film may not be homogeneous, particularly for alloy growth, because the vapor pressures of the source materials are different at a given temperature and must be controlled carefully to produce a film of a desired composition when two or three sources and multi-temperatures are used. Also, the composition of the vapor may differ considerably from that of the source during the evaporation process, since the vapor may consist of elements together with compounds having stoichiometry different from the source, thereby resulting in deposits of non-stoichiometric composition.
As may be thus appreciated, it is highly desirable to have a process which provides (1) a fast rate of growth for IVA-VIA compounds on substrates, (2) multilayer film structures of different compositions and impurity levels and conductivity type without removing the structures from the growth system and (3) large area films of controlled compositions and stoichiometry.